1. Field of the Invention
The present invention relates to a control apparatus for controlling an operating stroke of a reciprocating member such as a spindle quill of tapping machine.
2. Discussion of the Prior Art
An apparatus for controlling a movement of a reciprocating member widely uses a relatively inexpensive encoder for detecting the position of the reciprocating member. For instance, a rotary encoder is used for controlling the operating stroke of a spindle quill of a tapping machine wherein the spindle quill is advanced and retracted while a main spindle is rotating in the forward and reverse directions, respectively, the perform a tapping operation. The advancing and retracting movements of the spindle quill are mechanically synchronized with the forward and reverse rotations of the main spindle (i.e., spindle quill). The rotary encoder generates pulse signals after the forward rotation of the main spindle is started. The control apparatus counts the number of the pulse signals. When the counted number of the pulse signals reaches a predetermined value, the rotating direction of the spindle is switched to the reverse direction, to retract the spindle quill to the original or home position. Namely, the conventional control apparatus is adapted to control the operating stroke or bottoming position of the spindle quill (i.e., a tap held by the quill), by directly counting the pulses generated by the rotary encoder.
In the above arrangement using the encoder, the minimum amount of movement of the spindle quill that can be controlled based on the pulses from the encoder is determined by or limited to the resolution of the encoder. Thus, the accuracy of controlling the operating stroke of the quill is not sufficiently high where the resolving power of the encoder is relatively low.
Consequently, the tapping tool held by the spindle quill may collide with the bottom of a blind hole to be tapped. In other words, the above arrangement does not permit the tap to cut an internal thread down to a point immediately above the bottom of the blind hole, without the tip of the tap bottoming the hole.
The control accuracy of the operating stroke may be improved by using an encoder whose resolution is sufficiently high. In this case, however, the encoder is expensive, and requires higher degree of assembling accuracy and a complicated control circuit for processing the pulse signals from the encoder, thereby increasing the cost of manufacture of the control apparatus.
Usually, an encoder is adapted to generate an A-phase signal, and a B-phase signal whose phase is shifted from the A-phase signal by an amount corresponding to 1/4 of the pulse interval, so that the direction of movement of the relevant reciprocating member is detected. It is considered to divide a time length corresponding to the pulse interval, into four one-quarter fractions, by using the B-phase signal. To accurately detect the times corresponding to these fractions of the pulse interval, not only the A-phase signal should rise so as to exactly establish the nominal pulse interval, but also the A-phase signal should fall and the B-phase signal should rise and fall to exactly establish the pulse duration. Although the pulse interval of the A-phase signal is held comparatively constant, the pulse durations of the A-phase and B-phase signals tend to fluctuate, particularly where an inexpensive encoder is used.